Volume 118, Issue 8, 22 February 2003
 COMMUNICATIONS


A scaling principle for the dynamics of density fluctuations in atomic liquids
View Description Hide DescriptionOn the basis of the fully renormalized kinetic theory of equilibrium classical atomic fluids, we conjecture that different fluids with very different interatomic potentials might have very similar dynamical density fluctuations if they have similar static pair correlation functions.Molecular dynamics simulations are presented that provide support for this conjecture. This suggests that in order to understand how the interatomic potential of a liquid determines its dynamical properties, it is worthwhile focussing separately on the two questions of how the potential affects the pair correlation function and how the pair correlation function affects the dynamics.

Nuclear magnetic resonance noise spectroscopy using twophoton excitation
View Description Hide DescriptionTwophoton excitation is employed in conjunction with stochastic nuclear magnetic resonance(NMR) at high magnetic field to acquire pulsed NMRspectra with high sensitivity in the absence of an onresonance radiofrequency excitation. These conditions allow the NMR receiver to be active during excitation and therefore eliminate the receiver deadtime. Examples are presented demonstrating the direct observation of nuclear magnetization under nutation and spinlocking conditions. Applications of the techniques are discussed.

Free energy barrier to melting of singlechain polymer crystallite
View Description Hide DescriptionWe report Monte Carlo simulations of the melting of a singlepolymer crystallite. We find that, unlike most atomic and molecular crystals, such crystallites can be heated appreciably above their melting temperature before they transform to the disordered “coil” state. The surface of the superheated crystallite is found to be disordered. The thickness of the disordered layer increases with superheating. However, the order–disorder transition is not gradual but sudden. Freeenergy calculations reveal the presence of a large freeenergy barrier to melting.
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 ARTICLES

 Theoretical Methods and Algorithms

Molecular vibrations: Iterative solution with energy selected bases
View Description Hide DescriptionAn efficient and accurate quantum method for the calculations of many large amplitude vibrational states of polyatomic molecules is proposed and tested on three triatomic molecules; and HCN. In this approach we define zeroorder reduced dimensional Hamiltonians using minimum energy reduced dimensional potentials. The eigenfunctions and eigenvalues of and are used to form an energy selected basis (ESB) for the full system including all the product functions for which We show that ESB can be used efficiently in an iterative solution of the Schrödinger equation by the transformation between the ESB and the direct product quadrature grid. Application of the ESB of onedimensional basis functions is shown to be very efficient for vibrational states of and up to 30 000 and 23 000 cm^{−1}, respectively. A combined twodimensional/onedimensional basis is used very effectively for HCN above the isomerizationenergy to HNC. The present approach is shown to be substantially more efficient than either the direct product discrete variable representation (DVR) bases or compact bases from the DVR with the sequential diagonalization/truncation method.

Multichromophore excitons and resonance energy transfer: Molecular quantum electrodynamics
View Description Hide DescriptionResonance energy transfer in multichromophore arrays, such as lightharvesting complexes and dendrimers, is well documented. The theory involved in the migration of energy to an acceptor from one excited donor, or concertedly from two of three such donors, has also been thoroughly investigated. In cases where the initial excitations form a delocalized exciton amongst the donors the corresponding theory describing transfer to an acceptor is less welldeveloped. By considering a model dendrimeric system we analyze the configuration and energy transfer properties of excitonic states formed by the absorption of one and two photons. Using molecular quantum electrodynamics and interactionpair notation we quantify these effects in terms of quantum amplitudes. New insights result from our description in terms of state sequences. In particular it is possible for the first time to identify quantum interconnections between different exciton relaxation routes.

Correlation between electronic and molecular structure distortions and vibrational properties. I. Adiabatic approximations
View Description Hide DescriptionDistortions of the electronic and molecular structures of a polarizable molecule in an inhomogeneous electrostatic potential were investigated by using perturbation theory. The secondorder electronic energy correction term is directly related to the charge response kernel, which is a site representation of the nonlocal charge response susceptibility. Instead of using the sumoverstate expression for the charge response kernel, we discuss how a finitefield method using point charges can be used to calculate the charge response kernel. By invoking various levels of adiabatic approximations, four different coupled differential equations for the vibrational wave functions were obtained. From the effective vibrational potential functions thus obtained, the molecular structural distortion induced by the external electrostatic potential was shown to be calculable. We also present a discussion on how vibrational properties are affected by the presence of the electrostatic potential. The relationship between the vibrational frequency shift and molecular structural distortion, when a polarizable molecule is exposed to an electrostatic potential, was elucidated.

Correlation between electronic and molecular structure distortions and vibrational properties. II. Amide I modes of complexes
View Description Hide DescriptionHydration effects on the molecular structure and amide I mode frequency of a prototype peptide molecule, Nmethylacetamide (NMA), when it is solvated by a few water molecules, were investigated by carrying out ab initio calculations for a number of NMA–water complexes. The harmonic frequency shift of the amide I mode in complex was found to originate from the combination of the molecular cubic anharmonicity and displacement of the amide I coordinate when the NMA is hydrated. Using a multivariate leastsquare fitting method, the effective transition charges of six NMA sites were determined. A brief discussion on how this empirical model can be used to quantitatively describe solvatochromic frequency shift of the NMA amide I mode in solution is presented.

Cooperative effects in onedimensional chains of threecenter hydrogen bonding interactions
View Description Hide DescriptionCooperative effects in a onedimensional network of intermolecular bifurcated hydrogen bondinginteractions are investigated by means of ab initio calculations. The trans–transconformation of the diformamide molecule is used as a basic motif to model a chain of bifurcated H bonds. In this model system, the two proton–acceptor atoms belong to the same molecule. The onedimensional network is modeled then by periodically stacking up to 12 molecules of the unit motif. Different indicators of Hbond strength such as energetic, structural, dielectric, vibrational frequencies, and isotropic chemicals shifts consistently show significant cooperative effects in the chains. The dissociation energy in the dimer is calculated to be 9.88 kcal/mol, while that of the strongest interaction in the decamer is calculated to be 26.12 kcal/mol (164% increase in cooperativity). Thus, although threecenter H bonds can be viewed as a consequence of proton deficiency, in some cases they may also be viewed as the natural result of an interaction that is itself energetically favorable and capable of competing with the more conventional twocenter H bonds. Natural bond orbital analysis reveals substantial charge delocalization within each molecule, and charge transfer along the chains. Interestingly, this charge delocalization makes the system a good candidate for resonanceassisted H bonding which in turn increases the covalent character of this type of bifurcated Hbondinginteraction.

Performance of coupled cluster theory in thermochemical calculations of small halogenated compounds
View Description Hide DescriptionAtomization energies at 0 K and heats of formation at 298 K were obtained for a collection of small halogenated molecules from coupled clustertheory including noniterative, quasiperturbative triple excitations calculations with large basis sets (up through augmented septuple zeta quality in some cases). In order to achieve near chemical accuracy (±1 kcal/mol) in the thermodynamic properties, we adopted a composite theoretical approach which incorporated estimated complete basis set binding energies based on frozen core coupled clustertheoryenergies and (up to) five corrections: (1) a core/valence correction; (2) a Douglas–Kroll–Hess scalar relativistic correction; (3) a firstorder atomic spin–orbit correction; (4) a secondorder spin–orbit correction for heavy elements; and (5) an approximate correction to account for the remaining correlation energy. The last of these corrections is based on a recently proposed approximation to full configuration interaction via a continued fraction approximant for coupled clustertheory [CCSD(T)cf]. Failure to consider corrections (1) to (4) can introduce errors significantly in excess of the target accuracy of ±1 kcal/mol. Although some cancellation of error may occur if one or more of these corrections is omitted, such a situation is by no means universal and cannot be relied upon for high accuracy. The accuracy of the Douglas–Kroll–Hess approach was calibrated against both new and previously published fourcomponent Dirac Coulomb results at the coupled cluster level of theory. In addition, vibrational zeropoint energies were computed at the coupled cluster level of theory for those polyatomic systems lacking an experimental anharmonic value.

Modeling chemical reactions for conformationally mobile systems with force field methods
View Description Hide DescriptionA previously proposed method for modeling transition structures as minima on the seam of two force field energy functions, denoted SEAM, is analyzed with respect to improvements in the functional form of key energetic terms and parameters. It is shown that the SEAM method is capable of accurately reproducing transition structures from electronic structure methods, at a fraction of the cost. The SEAM method can be used to automatically sample different conformational transition structures for a given reaction. Depending on the underlying force field and the reaction, the relative energies of the transition structures may or may not be sufficiently accurate for selecting the important lowenergy structures. A combination of using the SEAM method for determining geometries and a lowcost electronic structure method for single point energy calculations appears to be an attractive combination for identifying lowenergy transition structures for systems having many degrees of freedom.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

On the efficient representation of comprehensive, precise spectroscopic data sets: The state of
View Description Hide DescriptionMixed representations—polynomials in at low neardissociation expansions (NDEs) in at high —are tested for their ability to fit a comprehensive and precise data set for the A state of The combined functions for the vibronic energy and the rotational constant are rendered smooth at the point of switchover through two approaches: the use of Lagrange’s method of undetermined multipliers to incorporate exactly satisfied constraints for continuity in the functions and their first derivatives, and the use of a smooth switching function. As compared with a previously reported pure NDE analysis [Appadoo et al., J. Chem. Phys. 104, 903 (1996)], both approaches yield significantly reduced chisquare and a more realistic extrapolation of from the highest analyzed level to dissociation. The switchingfunction method has a number of advantages over the constraint method, and is thus recommended as the preferred approach for fitting to mixed representations.

Rotationvibration states of at dissociation
View Description Hide DescriptionCalculations are presented which estimate energies for all the bound rotationvibration energy levels of with rotation angular momentum 2 and 8. The calculations, which use Radau coordinates with axis of the molecule embedded perpendicular to the molecular plane, are performed on 128 nodes of a massively parallel computer. It is found that convergence with respect to basis set size of the higher states is fairly slow and that further improvements are beyond the capabilities of the current computational setup.

Emission spectra of TiH and TiD near 938 nm
View Description Hide DescriptionHigh resolution, near infrared emission spectra of TiH and TiD have been recorded with a Fourier transformspectrometer. The TiH and TiD molecules were made in a titanium hollow cathode lamp operated with a mixture of neon and hydrogen or deuterium gases. A heavily perturbed band system near 938 nm is assigned as a electronic transition. Line positions and approximate spectroscopic constants are reported for the 0–0 band of the new transition. The TiH transition might potentially be observed in late Mtype stars and in Ltype brown dwarfs.

Rotational spectra of and the isotopic species of
View Description Hide DescriptionTwo new carbon chain radicals terminated with a nitrile group, and have been detected in a supersonic molecular beam by Fourier transformmicrowave spectroscopy. In addition, at least three hyperfinesplit rotational transitions of the singlysubstituted isotopic species of have also been observed. Both and are linear chains with electronic ground states, and both radicals have resolvable hyperfine structure and lambdatype doubling in their lowest rotational levels. At least four transitions in the lowestenergy fine structure component were measured between 7 and 22 GHz for both molecules, and at most nine spectroscopic constants were required to reproduce the measured spectra to a few parts in Precise sets of rotational, centrifugal distortion, spinrotation, and hyperfine coupling constants were also determined for the isotopic species of by combining the centimeterwave measurements here with previous millimeterwave data. The hyperfine coupling constants of isotopic differ from those of the isoelectronic chain but are fairly close to those of isovalent indicating a nearly pure electronic ground state for Although the strongest lines of are more than five times less intense than those of owing to large differences in the ground state dipole moments, both new chains are more abundant than Searches for have so far been unsuccessful. The absence of lines at the predicted frequencies implies that the product of the dipole moment times the abundance is more than 60 times smaller for than for suggesting that the ground state of may be for which the dipole moment is calculated to be small.

Structures and stabilities of small silicon clusters: Ab initio molecularorbital calculations of
View Description Hide DescriptionAb initio allelectron molecularorbital calculations have been carried out to study the structure and relative stability of small siliconclusters A number of lowenergy geometric isomers are optimized at the secondorder Møller–Plesset (MP2) level. Harmonic vibrational analysis has been performed to assure that the optimized geometries are stable. The total energies of stable isomers are computed at the coupledcluster single and double substitutions (including triple excitations) [CCSD(T)] level. The calculated binding energies per atom at both the and levels agree with the experiments. For and the lowestenergy structures are the same as those predicted previously from the allelectron optimization at the Hartree–Fock (HF) level [Raghavachari and Rohlfing, J. Chem. Phys. 89, 2219 (1988)]. For the lowestenergy isomer is same as that predicted based on densityfunctional planewave pseudopotential method [Vasiliev, Ogut, and Chelikowsky, Phys. Rev. Lett. 78, 4805 (1997)]. Particular attention has been given to because several lowenergy geometric isomers were found nearly isoenergetic. On the basis of calculation, we identified that the isomer, a tricapped trigonal prism with two additional caps on side trigonal faces, is most likely the globalminimum structure. However, another competitive geometric isomer for the global minimum is also found on basis of the calculation. Additionally, calculations of the binding energy and the clusterpolarizability offer more insights into relatively strong stability of two magicnumber clusters and

Ionization energies and spatial volumes of the singly occupied molecular orbital in hydrated magnesium clusters
View Description Hide DescriptionThe vertical and adiabatic ionization energies as well as the spatial volumes of the singly occupied molecular orbital (SOMO) of were determined by ab initio calculations.Ionization energies were evaluated from Koopmans’ theorem and explicitly as differences of the total energies of and as obtained by Hartree–Fock, postHartree–Fock and gradient corrected density functional(DFT) methods. In the case of clusters with a sixfold coordinated magnesium cation Koopmans’ theorem fails for In contrast this is a valid approximation for all other cluster sizes. The most stable isomers of exhibit significantly enhanced SOMO volumes. This coincides with a significant drop in ionization energies and with an increase in electron correlation. In these clusters Koopmans’ theorem is a crude approximation due to the neglect of electron correlation. The cluster size dependency of orbital relaxation and change in electron correlation upon ionization allows for an analytical fit in terms of the spatial SOMO volume. Reorganization energies and SOMO volumes indicate strong structural changes in the clusters during ionization due to a significant localization of the SOMO in and

Short Hbonds and spontaneous selfdissociation in Effects of Hbond topology
View Description Hide DescriptionThere are 30026 symmetrydistinct ways to arrange 20 water molecules in a dodecahedral cage with nearly optimum hydrogen bond lengths and angles, analogous to the arrangements that give rise to the zeropoint entropy in iceIh. The energy of hydrogen bond isomers in assumed to be similar in the past, differs by up to 70 kcal/mol. The isomers differ widely in their hydrogen bond lengths, some exhibiting bond lengths as short as ∼2.4 Å. The differences among the isomers extends to their chemical properties: In some arrangements one or more water molecules spontaneously selfdissociate, giving rise to spatially separated excess proton and hydroxyl ion units in the cluster. Isomers that exhibit these unusual properties can be identified by features of their hydrogen bondtopology.

The absorption spectroscopy of the lowest pseudorotational states of tetrahydrofuran
View Description Hide DescriptionThe rotational structure of the pseudorotational (PR) band has been observed in jetcooled tetrahydrofuran in the 170–360 GHz frequency range. The observed transitions were analyzed together with the previously obtained microwave data of Meyer and coworkers [R. Meyer, J. C. Lopes, J. L. Alonso, S. Melandri, P. G. Favero, and W. Caminati, J. Chem. Phys. 111, 7871 (1999)]. The experimentally observed transitions provide direct spectroscopic evidence of the symmetry ordering of the lowest four observed PR states. Based on the symmetry properties of the pseudorotational states involved in this study, an analytical model of the potential energy surface (PES) along the pseudorotational path has been proposed that provides a consistent explanation of all the observed transition frequencies, including those from the early IR work. In addition, an analysis of the variation of the rotational constants of the molecule in different PR states has been performed using the proposed model. This analytical PES and the derived rotational constants are compared to the results of it ab initio calculations. A discussion of the results obtained by different methods is given.

Photodissociation of highly vibrationally excited in the region: Initial vibrational state dependence of N–H bond dissociation cross section
View Description Hide DescriptionUltraviolet photolysis of highly vibrationally excited in the band has been studied using a crossed laser and molecular beams method. Relative cross sections of N–H bonddissociation via the transition have been determined by the measurement of the action spectrum, utilizing a (2+1) resonance enhanced multiphoton ionization scheme of product H atoms. The obtained cross section for the photolysis of the state (4 quanta in the symmetric stretching and 1 quantum in the antisymmetric stretching) was 1.23±0.06 times as large as that of the state. To interpret the ratio of the obtained cross sections, we have evaluated the Franck–Condon factors for these vibrational states, by utilizing timedependent wavepacket calculations on the potential energy surfaces constructed by an ab initio molecular orbital procedure.

Infrared spectroscopy of nanoparticles flowing in a uniform supersonic expansion
View Description Hide DescriptionThe infrared signature of carbon dioxide clusters of nanometric size is discussed both in the bending mode at 15 μm) and in the asymmetric stretching mode at 4.2 μm) spectral region of the monomer. The carbon dioxidenanoparticles were formed using a capillary tube injection inserted upstream of a uniform supersonic flow of argon generated by a Laval nozzle. The size of the formed clusters was varied by changing the stagnation pressure of the capillary. The empirical power law connecting to the number N of monomers per cluster: was verified in this work. The cluster mean size was estimated using a Rayleigh scattering experiment showing the formation of nanometric clusters whose radii are in the range corresponding to The thermodynamic and kinetic parameters of the flow were determined from the rovibrational absorption lines of the monomer and from a timeofflight experiment. The measured flow velocity and flow temperature show that condensation is responsible for both a strong flow warming and a nonnegligible flow acceleration. The translational and rotational temperatures of the monomers were found to be identical, highlighting a thermal equilibrium between these two motions. The cluster temperature ranging from 93 to 135 K was estimated assuming a thermal equilibrium between the clusters and the monomer bath, induced by a high flow density of about The double peak feature at 657 and 667 cm^{−1} reflects the crystalline nature of the clusters. A single Lorentzian peak is observable at 2360 cm^{−1} whose position however appears to be weakly size dependent. The pronounced narrowing of the peak with increasing N surprisingly stopped evolving for and